Langerhans Cell Histiocytosis (LCH) is the most common of histiocytosis, that results from the accumulation of pathologically activated DC associated with granuloma like lesions that consist of T cells and macrophages leading to irreversible tissue damage. Clinical approaches to LCH remain empiric due to poor understanding of LCH biology. During the previous budget period, we established the pivotal functional role of the BRAFV600E mutation in LCH pathogenesis and discovered the multiple origin of the LCH cell. In contrast to the dogma that suggests that LCH derive from pathological epidermal Langerhans cells (LCs), we showed that the LCH cell can derive from V600E mutated hematopoietic progenitors or differentiated DC and that occurrence of the V600E mutation along the DC lineage defines clinical risk in LCH patients. In addition, we generated the first mouse model of LCH-like disease allowing us to characterize the molecular mechanisms that drive LCH. These studies revealed the potential key contribution of physiological ERK activation in DC differentiation, and induction or expansion of Tregs, which may explain why Treg accumulate in high numbers in LCH lesions. These studies also established the dual contribution of sustained ERK activation and extracellular cues provided by T cells to LCH pathogenesis. Thus the goal of this application is to characterize the role of physiological ERK activation in DC and Treg homeostasis in vivo and how sustained ERK activation contributes to LCH pathogenesis. We will also dissect the contribution of cell extrinsic signals to the transformation of pathologically activated DC and to LCH growth. Based on the results of the mechanistic studies we will develop novel combination strategies that target cell intrinsic and extrinsic cues shown to modulate LCH pathogenesis. Specific Aim 1: To characterize the physiological role of the ERK pathway on DC differentiation, DC homeostasis and induction or expansion of T regulatory cells in situ. Specific Aim 2: To determine the contribution of sustained ERK-activation as well as cell extrinsic cues to LCH pathogenesis Specific Aim 3: To test combination strategies that target the ERK pathway along with cell extrinsic cues for the treatment of BRAFV600E-driven LCH lesions.